Palladium and palladium alloys are used in a number of applications because of their chemical inertness, hardness, excellent wearability, bright finish and high electrical conductivity. In addition, they do not form oxide surface coatings that might increase surface contact resistance. Particularly attractive is the use of palladium alloys as electrical contact surfaces in the electrical arts such as in electrical connectors, relay contacts, switches, etc.
Electrical contact manufacture advantageously employs a "stripe-on-stripe " processing. A metal strip, typically a copper bronze material, is coated with a stripe of a metal. To reduce an expense of precious metals the stripe is produced only on those portions of the strip which when subsequently formed into an electrical connector will be subjected to extended wear and requires superior electrical connection characteristics. Following the coating application, the metal strip is subjected to stamping and forming operations.
The process of coating the strip with a stripe of contact material can be performed in several ways including an inlaying method and an electroplating method. The inlaying method calls for metal cladding of a metal substrate with an inlay of a noble metal or alloy. In the inlaying method a strip of a substrate metal is inlayed with a stripe of an alloy followed by capping with gold. For example, a strip of copper-bronze alloy is inlayed with 40/60 Ag/Pd alloy about 90 microinches thick followed by a 10 microinch thick Au capping. The inlayed strip is then stamped and formed into a connector. The alloy material is expensive and, unfortunately, the inlayed stripe wears out faster than is desirable. The electroplating method consists of electroplating a strip of the copper bronze substrate with a stripe of protective coating, including electrodeposition of Pd alloyed with Ni or Co, followed by Au capping, typically in a reel-to-reel operation. A suitable process for electroplating palladium and palladium alloys from an aqueous solution is described in a number of U.S. patents granted to J. A. Abys and including U.S. Pat. No. 4,468,296 issued on Aug. 28, 1984; U.S. Pat. No. 4,486,274 issued on Dec. 4, 1984; and U.S. Pat. Nos. 4,911,798 and 4,911,799, both issued on Mar. 27, 1990, each of which is incorporated herein by reference. The stripe-coated strip is then subjected to the stamping and forming operation. The total amount of precious metals deposited in the electroplating process is small and the process is less costly than the inlaying process. Therefore, a device with an electrical contact produced with electroplated stripe would be less costly than with the inlayed stripe, even if being equal in other aspects.
Applicants have observed, however, that electrodeposits of alloys, for instance hard gold, palladium nickel or palladium cobalt alloy, exhibited undesirable cracking defects when subjected to the forming operation as required in the production of such devices. Therefore, it is desirable to alleviate these undesirable characteristics of the electroplated palladium alloy stripe.